A Cone Shaped Tapered Chamber for Antenna Measurements Both in Near Field and Far Field in the 200 MHz to 18 GHz Frequency Range and Extension of the Quiet Zone using an RF Lens

Abstract

Traditionally, tapered chambers are constructed using a square based pyramidal shaped taper. The taper is then shaped into an octagon and finally transformed into a cylindrical launch section. This approach is related to the manufacturability of different absorber cuts. This paper introduces a chamber where the conical shape of the launch in continued throughout the entire length of the tapered chamber. The results of the free space VSWR test over a 1.5 m diameter quiet zone (QZ) are presented at different frequencies. The conical taper appears to have a better illumination wave front and better QZ levels even at frequencies above 2 GHz than the standard traditional approach. This design was implemented in Singapore and the actual chamber was designed to have a secondary near field range for planar and spherical scans. As with all antenna chambers, as the frequency increases, the usable or far field illuminated QZ is reduced. At a 12 m distance from the feed to the turntable, the quiet zone at 8 GHz is reduced to 45 cm. The chamber includes a near to far field range to allow the use of the chamber at higher frequencies when testing electrically large antennas. Another solution implemented to extend the quiet zone at high frequency is to use a large dielectric lens to improve the phase distribution of the field. A light weight, broadband lens with a diameter of 2 m was developed by Matsing Pte Ltd. The parameters of the lens were specially customized for the tapered chamber built. The RF lens, weighing just 35 kg, has a focal length of 10 m. It was installed in front of the turn table. The performance of the tapered chamber with the RF lens is presented. The usable far field QZ was increased by using a dielectric RF lens that allows for electrically larger antennas to be measured in the tapered range of the chamber.